Forest Structure Affects the Stoichiometry of Periphyton Primary Producers in Mountain Streams of Northern Patagonia

Abstract

Riparian zones are major components of stream ecosystems that influence the physical, chemical, and biological parameters. In particular, the distribution of vertical foliage and the structure of riparian vegetation determine light availability in canopied streams. Here, we analyzed how forest structure will modify light availability and thus affect primary producers’ photosynthetic parameters and the periphyton stoichiometry of mountain streams. We carried out field sampling in four streams with different canopies located in the North-Patagonian Andes and conducted a field experiment in which light conditions were manipulated for four months. Then, we linked our results to qualitative climate change scenarios for North-Patagonian forest predicting how future climate change will affect primary producers and periphyton stoichiometry in low-order streams through modifications in the structure of canopied zones. Finally, we found that biomass, photosynthetic parameters and the elemental content of periphyton exhibited a bell-shaped relationship with light availability which was, in turn, dependent on canopy cover. These trends are characterized by an increase from low light up to 250 μmol m−2 s−1 and a decline when light is over 750 μmol m−2 s−1. Thus, intermediate light resulted in optimal conditions for primary producers’ photosynthesis; however, these intermediate canopied zones are predicted to decrease in the future. Therefore, we predict changes in stream ecosystem stoichiometry due to variations in primary producers’ photosynthesis, and, consequently, periphyton elemental composition as an outcome of forest structure modifications due to climate change.